Term for an organism whose cells are derived from at least two genetically different zygote lines (diploid cells). In botany this is used for those plants whose body has mutated cell lines in different tissues or organs. This relatively common phenomenon is known from ornamental plants, where sectoral chimeras with multicoloured petals or elongated striped, white-green or purple-green-white leaves occur. Periclinal chimeras are plants whose external epidermal tissue deviates from the genotype of the internal cell lines by mutation, whereby the mutations in both cell lines occur independently of each other and at different gene locations. Due to the double set of chromosomes of the grapevine, a gene locus can mutate into four different alleles.
Somatic chimeras are the normal case, particularly in old, widespread and often vegetatively duplicated varieties, because the two basic cell lines L1 (epidermal cells) and L2 (inner cells) were spatially separated from each other as soon as the embryo differentiated in the semen and were thus able to mutate independently from each other over centuries. In ancient varieties such as Pinot and Traminer (see there in detail), mutations have led to the formation of numerous somatic chimeras, which can be genotypically differentiated by one, several or all four possible alleles of a gene locus. These are also known as shoot or bud mutations, because a whole new shoot with the altered or new somatic and thus visible characteristics grows from the buds through cell division of the mutated stem cells.
In the vine, a periclinal mutation in the epidermis line L1 can be expressed, for example, by increased hairiness as in the two varieties Pinot Meunier (black Riesling) or Garnacha Peluda (Lladoner Pelut). There are also so-called mericlinal chimeras such as Pinot Gris, on whose grey grapes fruit branches with white berries can appear. The Tressot Panaché is a classic sectoral chimera with white and blue sectors on the skin of individual berries, which appear either white, blue or in sectors white-blue striped. However, most mutation events are externally not or only gradually recognizable. For example, it may be a slight shortening of the ripening period, a darker berry colour or a more intense taste. The fixation of such a clone mutant includes the vegetative propagation of the mutated axillary shoot to an independent and further propagated vine. After more precise analyses, most clones of Pinot Noir turned out to be periclinal chimeras, which was to be expected, since it is extremely unlikely that a spontaneous mutation event occurs in both cell lines at the same place on the DNA strand.
In the case of such genotypically characterized and distinguishable clone variants, it is better to speak of clone mutants, because a clone in the strict sense of the word is initially only a copy of the original, which only develops into a differentiable clone in the viticultural sense through mutations. Since adventitious buds and water shoots only develop from cell lines of the L2-layer, somatic chimeras can change without additional mutation events, solely by vegetative propagation from adventitious shoots. The resulting plants then only show the mutation history of the L2 cell line. In summary, it can be said that a chimera always results from a mutation. However, not every mutation can be called a chimera, although this is the rule for vines. A grafted vine (root with graft) is also a chimera, in this case it is a mesoclinal chimera (upper and lower region). See also molecular genetics for more information on this topic.
Pictures: Ursula Brühl, Doris Schneider, Julius Kühn Institute (JKI)